The present invention relates to methods for making high temperature coatings comprising precursor polymers which, upon pyrolysis or other energetic treatment, decompose to yield substantially pure refractory metal carbides and/or refractory metal borides.
Considerable effort has been devoted over the past 15 years to the development of effective methods for manufacturing ceramic matrix composites (CMC""s). Several approaches with potential for industrial use have been identified. The development of CMC""s with high temperature stability theoretically is possible; however, CMC""s have not yet been developed for use in extremely high temperature applications, such as multistage nozzles for rocket motors. Such nozzles must be capable of exhibiting high strength even after repeatedly withstanding temperatures of 1600xc2x0 C. and even higher.
Currently, multistage nozzles are made from tungsten and graphite, which have relatively high melting/sublimation pointsxe2x80x94a 3410xc2x0 C. melting point for tungsten, and a 3650xc2x0 C. sublimation point for graphite. The high temperature strength of a material is proportional to the melting point of that material. If CMC""s could be made using materials with higher melting/sublimation points than tungsten and graphite, then the resulting CMC""s should be effective alternative materials for making high temperature components, such as multistage nozzles.
Certain metal carbides and metal borides have melting temperatures even higher than the melting/sublimation points of tungsten and graphite. For example, hafnium carbide has a melting temperature of 3890xc2x0 C. and tantalum carbide has a melting temperature of 3880xc2x0 C. Metal carbides also exhibit desirable brittle to ductile transition temperatures in the range of 1725-1980xc2x0 C.
A CMC having a matrix of a refractory metal carbide and/or metal boride and comprising between about 20-30% particulate silicon carbide theoretically would be an ideal alternative for tungsten and graphite in multistage nozzles. Such metal carbides and/or metal borides also might be useful as high temperature coatings for other surfaces which are exposed to high temperatures during operation. In fact, the United States Air Force has recently initiated a new programxe2x80x94Integrated High Pay-Off Rocket Propulsion Technology (IHPRPT)xe2x80x94to incorporate such advanced materials into rocket and space propulsion systems.
Unfortunately, the most widely used method for making CMC""sxe2x80x94chemical vapor infiltration (CVI)xe2x80x94is slow, complex, and has many inherent difficulties. One major difficulty for high temperature applications is that CVI produces a CMC with substantial residual porosity (15-25%). The greater the porosity, the lower the strength of the CMC.
Polymer infiltration/pyrolysis (PIP) can produce a less porous CMC. However, PIP can only be used to make metal carbide/metal boride CMC""s if precursor polymers are developed which will decompose upon pyrolysis or other energy treatment to yield substantially pure metal carbides and metal borides.
The present invention provides a method for making a high temperature coating comprising, applying to a surface a precursor polymer selected from the group consisting of a polymer of organo-transition metal complexes selected from the group consisting of transition metal coordinated olefinic groups, selected from the group consisting of allyl groups, vinyl groups, and olefinic groups comprising from about 2 to about 8 carbon atoms, a polymer of molecules selected from the group consisting of boranes, carboranes, and combinations thereof bonded together via an organometallic moeity comprising a transition metal and a ligand, the molecules being selected from the group consisting of ortho-deca-carboranes, meta-deca-carboranes, closo-boranes, nido-boranes, arachno-boranes, hypho-boranes, conjuncto-boranes, and combinations thereof, and a polymer comprising organometallic hydrides comprising a transition metal immobilized on a plurality of unsaturated bonds in a backbone polymer; and exposing the precursor polymer to conditions effective to decompose the precursor polymer to a product selected from the group consisting of a refractory metal boride and a refractory metal carbide.
Synthetic inorganic and organometallic chemistry has been used to produce a variety of metal-containing polymer species which, upon pyrolysis or other energetic treatment, decompose to yield substantially pure metal carbides and/or metal borides. Two different approaches were used to obtain such organometallic precursor polymers.
In a first embodiment, a transition metal salt is mixed with one or more organometallic(s) containing at least one unsaturated carbon-carbon bond to form organo-transition metal complexes, which are polymerized to form the precursor polymer. This embodiment has the advantage of guaranteeing that each unit of monomer will contain a metal atom. One disadvantage of this embodiment is that it does not produce high molecular weight precursor polymers.
High molecular weight precursor polymers are advantageous for use in a PIP process because high molecular weight precursor polymers tend to produce higher ceramic yields. Unfortunately, the viscosity of a polymer also increases with the molecular weight of the polymer. Precursor polymers with lower viscosity are preferred for an ideal PIP process. This inherent conflict may be resolved by using high molecular weight precursor polymers having relatively low viscosity, preferably a viscosity similar to a warm honey-like consistency. In order to produce such precursor polymers, the organo- transition metal complexes described above are polymerized with other comonomers which have low tendency to increase viscosity, as described in more detail below.
Preferred organometallics for use in this first embodiment include, but are not necessarily limited to metal coordinated substituted and unsubstituted allyl and vinyl organometallics comprising in the range of from about 2 to about 8 carbon atoms, preferably in the range of from about 2 to about 4 carbon atoms. Suitable allyl organometallics include, but are not necessarily limited to 1-methyl-2-propenyl magnesium chloride, 1-methyl-2-propenyl-magnesium bromide, 2-methyl-1-propenyl magnesium chloride, 1-methyl-2-propenyl-magnesium bromide, allyl magnesium chloride, allyl magnesium bromide. Suitable vinyl organometallics include, but are not necessarily limited to substituted and unsubstituted: vinyl lithium chlorides; vinyl magnesium chlorides; vinyl magnesium bromides; and similar compounds. Such compounds are available from Aldrich Chemical Co. A preferred organometallic is allyl magnesium bromide.
The organometallic should be reacted with a salt of a transition metal, defined herein as a transition metal selected from the group consisting of hafnium, tantalum, zirconium, titanium, vanadium, niobium, chromium, molybdenum, and tungsten. Preferred transition metals are selected from the group consisting of tantalum, hafnium, and zirconium. Such salts include but are not necessarily limited to metal halides, metal nitrates, metal sulfates, and metal acetates, with preferred salts being hafnium and tantalum chloride. Hafnium and tantalum chloride, and other metal halides, are available from a number of chemical sources. For example, hafnium chloride is available from Advance Research Chemicals, Inc., Catoosa, Okla., and Teledyne Wah Cheng, Albany, Oreg. Hafnium boride is available from Noah Chemical, Div. Noah Technologies Corp., San Antonio, Tex. Hafnium bromide and tantalum bromide are available from Wilshire Chemical Co., Inc., Gardena, Calif. Tantalum chloride is available from several sources, including Aithaca Chemical Corp., Uniondale, N.Y. and Trinitech International, Inc., Twinsberg, Ohio.
In a preferred embodiment, the precursor polymer is formed by suspending hafnium or tantalum chloride in a suitable organic solvent, preferably dry ether, and chilling to a temperature in the range of from about xe2x88x9270xc2x0 C. to about xe2x88x9290xc2x0 C., preferably about xe2x88x9278xc2x0 C. An excess of organometallic is added over a period of about 5 minutes. The excess preferably should be a slightly more than a ratio based on the number of halogen atoms in the transition metal salt. For example, if the transition metal salt contains four chloride atoms, then an excess of organometallic of just over about 4:1 is preferred. If the salt contains five chloride atoms, then an excess of organometallic of just over about 5:1 is preferred, etc.
The resulting solution, which typically will have an orange (hafnium) or green (tantalum) color, is stirred under an inert gas besides nitrogen, preferably argon, for a time in the range of from about 10 to about 20 hours, preferably about 16 hours, and the product is isolated by filtration through a suitable medium, such as filter paper. In a preferred embodiment, using allyl magnesium bromide, the product is allyl hafnium or allyl tantalum, which converts to the respective carbide in substantially pure form upon pyrolysis.
If it is necessary to increase the molecular weight of the precursor polymer, comonomers may be copolymerized with the foregoing organo-transition metal complexes during the same reaction. Suitable comonomers include, but are not necessarily limited to styrene, vinyl and divinyl benzene, and alkadienes having a number of carbon atoms in the range of from about 4 to about 14.
In an alternate reaction, the xe2x80x9corganic compoundsxe2x80x9d are boranes and carboranes, preferably ortho-deca-carboranes (C2B10H12). The carboranes are polymerized by reaction with organometallic halides to form what is believed to be the following: 
To prepare these precursor polymers, ortho-carborane, which may be obtained from Aldrich Chemical Co., should be lithiated, preferably by reaction with butyl lithium at about xe2x88x9278xc2x0 C. for a time in the range of from about 1 to about 3 hours. About 0.5 equivalents of a suitable organometallic halide, preferably bis(pentamethyl cyclopenta-dienyl)hafnium dichloride, should be added to the above prepared solution of the lithiated ortho-carborane at about xe2x88x9278xc2x0 C. and the solution should be slowly warmed to room temperature.
Other suitable commercially available boranes and carboranes include, but are not necessarily limited to meta-deca-carborane and closo-, nido-, arachno-, hypho-, and conjuncto-boranes, which could be deprotonated to the sodium, lithium, or potassium salt using techniques well known to persons of ordinary skill in the art. Boranes are widely commercially available, for example, from Aldrich Chemical Co. and from Fluka Chemical Co. Ortho-deca-carborane and meta-deca-carborane are commercially available from Aldrich Chemical Co.
Suitable ligands for the metal halide include, but are not necessarily limited to phosphines, amines, imines, sulfur-containing ligands, and cycloalkenyl groups. Preferred metal halides, which minimize the chance of adding impurity to the resulting borides, have the metal bound to at least one, preferably bound to two cycloalkenyl groups. Preferred cycloalkenyl groups are pentamethyl cyclopentadienyl groups.
Introduction of Organometallic Moieties into Preformed Polymers
In a second embodiment, organometallic moieties are immobilized on (or xe2x80x9cgrafted toxe2x80x9d) functional groups in a preformed backbone polymer. Suitable backbone polymers for use in this embodiment have repeated double bonds, and include, but are not necessarily limited to olefins, nitrites, acids, and ketones. In this embodiment, the molecular weight of the resulting polymer is dictated by the molecular weight of the backbone polymer.
Suitable backbone polymers include a broad range of molecular weights, preferably in the range of from about 1500 to about 7000 number average molecular weight (Mn). In a preferred embodiment, the backbone polymers include, but are not necessarily limited to heteroatom free polyalkadienes, heteroatom free polystyrene polyalkadiene block copolymers (PSPB""s), and isoprene polymers. Suitable PSPB""s and polyalkadienes are widely commercially available. Suitable commercially available isoprene polymers include, but are not necessarily limited to polybutadiene-isoprene, poly(isoprene), and poly(isoprene-styrene), which are available from Polysciences, Inc. A preferred backbone polymer is polybutadiene, available from Phillips Chemical Co., Div. of Phillips Petroleum Co., Borger, Tex., and from E. L. Puskas Co., Akron, Ohio.
The backbone polymers possess alkene groups which are reactive to certain organometallic compounds, such as those containing metal hydride (M-H) functions. The alkene bond will react with a metal hydride, incorporating the metal into the backbone polymer. Substantially any hydride comprising a transition metal selected from the group consisting of hafnium, tantalum, zirconium, titanium, vanadium, niobium, chromium, molybdenum, and tungsten should work in the invention. Preferred hydrides comprise a metal selected from the group consisting of hafnium, tantalum, and zirconium.
Preferred organometallic hydrides for use in this embodiment are bis(pentamethylcyclo-pentadienyl)hafnium dihydride, which may be obtained from Strem Chemicals, Inc. and dicyclopentadienyl tantalum trihydride. Organo-hafnium and tantalum dihydrides may be made from the respective commercially available chlorinated compounds as illustrated in the following equations wherein xe2x80x9cCp*xe2x80x9d stands for a pentamethyl cyclopentiadienyl compound:
Cp*2HfCl2+2BuLixe2x86x92Cp*2Hf(Bu)2
Cp*2Hf(Bu)2+H2xe2x86x92Cp*2HfH2
The Cp* hafnium dichloride and the lithiated butyl compound preferably should be mixed at a pressure of about 101.325 kPa (1 atm) and at a temperature in the range of from about xe2x88x9250xc2x0 C. to about xe2x88x9290xc2x0 C., preferably about xe2x88x9278xc2x0 C. and stirred for a period of time in the range of from about 15 minutes to about 3 hours, preferably for about 2 hours. The chloride atoms in the Cp* hafnium dichloride will be substituted by the butyl groups from the lithiated butyl compounds, resulting in dibutyl Cp* hafnium. The butyl Cp* hafnium then should be mixed with hydrogen gas at ambient temperature (typically in the range of from about 20 to about 25xc2x0 C.) and at ambient pressure (typically about 101.325 kPa or 1 atm) for a time period in the range of from about 10 hours to about 20 hours. The butyl groups will be substituted by hydrogen atoms to form dicyclopentadienyl hafnium dihydride.
To manufacture dicyclopentadienyl tantalum trihydride, tantalum (V) chloride is reacted with sodium Cp and sodium borohydride as follows:
TaCl5+2NaCp+NaBH4xe2x86x92Cp2TaH3
Pure Cp2TaH3, which can be obtained by sublimation, may be reacted with the backbone polymer, such as polybutadiene, under refluxing conditions or under high temperature (about 200xc2x0 C.) and high pressure (120 psi).
The foregoing reactions produce yellow or off-white precursor polymers, which are believed to have the following structure: 
Pyrolysis of the off-white or yellow precursor polymers at between about 1200-1400xc2x0 C. produces the respective metal carbide.
Polymer immobilized metal hydrides are sensitive to air and moisture and should be stored in an inert atmosphere, such as a dry-box, and transferred under a blanket of inert gas besides nitrogen, preferably argon.
Pyrolysis or Other Energy Treatment
The precursor polymers may be directly converted into high temperature coatings or the respective boride and/or carbide powders for use in making ceramic bodies, as fillers, etc. Upon pyrolysis, the precursor polymers of the present invention preferably decompose to a refractory metal carbide or a refractory metal boride comprising no more than xe2x80x9ctrace impurities.xe2x80x9d The term xe2x80x9ctrace impuritiesxe2x80x9d is defined herein as the minimum amount of impurities detectable by X-ray diffraction analysis. Quantitatively, the amount of impurities preferably is less than about 3 wt %, most preferably less than about 2 wt %.
In order to form a high temperature coating, the precursor polymer should be applied to a desired surface. Suitable surfaces comprise a wide variety of materials including, but not necessarily limited to, metals, alloys, intermetallics, ceramic matrix composites, metal matrix composites, polymer matrix composites, and polymers. The precursor polymer may be applied to the surface using any suitable method including, but not necessarily limited to, painting, spraying, and dipping. If needed, the precursor polymer may be mixed with a filler including, but not necessarily limited to one or more powder selected from the group consisting of ceramics, metals, and alloys. Once coated with the precursor polymer, the surface should be treated with an energy source sufficient to convert the precursor polymer to a refractory carbide or boride. Suitable energy sources include, but are not necessarily limited to pyrolysis and microwave treatment. In a preferred embodiment, the coated surface is fired to a temperature in the range of from about 800xc2x0 C. to about 1600xc2x0 C., preferably to about 1400xc2x0 C., for about two hours, or for a time sufficient to form a coating of refractory carbide or boride. The relatively low temperature required to perform polymer infiltration pyrolysis is energy efficient, and produces a coating with a level of porosity less than coating processed at higher temperatures. Surfaces bearing such coatings exhibit high temperature oxidation resistance. Depending upon the particular composition, coatings formed by the present invention are able to withstand repeated exposure to temperatures from about 3000xc2x0 to about 3800xc2x0 C.(hafnium chloride, m.p. 3890xc2x0 C.; hafnium boride, m.p. 3100xc2x0 C.; tantalum chloride, m.p. 3880xc2x0 C.; and tantalum boride, m.p. 3000xc2x0 C.).
In another preferred method, the precursor polymers are used to manufacture fiber-reinforced ceramic matrix composites (CMC""s) using polymer infiltration/pyrolysis (PIP), as described in J. R. Strife, et al. Ceram. Eng. Sci. Proc., 1999, 11, pp. 871-919, incorporated herein by reference. Basically, a plurality of plies of a suitable fibrous or woven material, typically about 6-7 plies, are laid up for infiltration by the precursor polymer. Suitable fibrous or woven materials include, but are not necessarily limited to carbon fibers, alumina fibers, ceramic fibers, silicon carbide, other carbides and nitrides, and the like. The reinforcement material may be continuous fiber, chopped fiber, woven cloth, or matted material. The precursor polymer is simply applied to the top of the stack of plies and allowed to infiltrate into the material. The infiltration should occur at ambient conditions; however, the assembly may be autoclaved to increase the speed of polymer curing, if desired.
After the precursor polymer has infiltrated the stack of plies and the stack has been autoclaved at a temperature of approximately 200xc2x0 C. for about 2 hours or less, the entire assembly should be exposed to energy sufficient to decompose the precursor polymer. Preferably, the assembly is subjected to pyrolysis at a temperature in the range of from about 800xc2x0 C. to about 1600xc2x0 C., preferably about 1400xc2x0 C. The pyrolysis preferably should take place under substantially any inert gas besides nitrogen, preferably argon due to its cost effectiveness. Although carbides and borides begin to form at about 800xc2x0 C., a temperature of about 1400xc2x0 C. is preferred because higher crystallinity is observed at about 1400xc2x0 C. These temperatures are sufficiently high to pyrolize the precursor polymers, but also sufficiently low to avoid disintegrating the plies or the fiber reinforcement material. The fibers remain substantially intact, resulting in a composite structure having greater strength than a composite pyrolyzed at temperatures greater than about 1400xc2x0 C.
The foregoing synthesis procedures are sufficiently flexible to provide a range of precursor polymers having a range of viscosities suitable for PIP, as well as a range of CMC""s formed using metal carbides and metal borides produced from such precursor polymers. If desired, fine ceramic, metal, alloy, polymer, and/or clay powder could be added to the polymer prior to PIP or other energy treatment to provide a wider range of product stoichiometry and greater process flexibility. The refractory metal carbides and metal borides should provide increased oxidation resistance.
The invention will be better understood with reference to the following examples, which are illustrative only, and should not be construed as limiting the present invention: